1. Field of the Invention
The present invention relates to a gas analyzer, and, more particularly, to a system for analyzing an impurity of ultra-low concentration, such as water, in a highly-purified gas.
2. Description of the Related Art
Known gas analyzers for analyzing an impurity of ultra-low concentration (for example, on the parts-per-billion, or ppb, level) include a dew-point meter, an atmospheric pressure ionization mass spectrometer (APIMS) and a plasma chromatography system. Such systems are especially useful when analyzing the water content of a purified gas.
Known dew-point meters are based upon detection of the frequency deviation of a quartz oscillator having an adsorbed water content, or the optical detection of moisture drops that have condensed on a mirrored surface. An example of the former type of dew-point meter is the AMETEK 5700 Moisture Analyzer; an example of the latter is disclosed on pages 41-42 of Ultra-Clean Technology, Vol. 1, No. 2.
Conventional dew-point meters are slow to respond to the change in dew point with respect to a change in moisture concentration at the ppb level (e.g., about -80.degree. C. at a freezing point), and thus cannot perform real-time analysis. See, for example, pages 13-21 of Ultra-Clean Technology, Vol. 1, No. 1. Further, the conventional dew-point meter system is large because it requires a helium refrigerator, as described in Ultra-Clean Technology, Vol. 1, No. 2, pages 41-42.
The conventional APIMS is highly sensitive, having an impurity detection limit of 1 part-per-trillion, or ppt (i.e., 1/10.sup.12), for a highly-purified gas. It can measure not only water content, but also such varied substances as oxygen and organic components simultaneously in real time. An example of an APIMS is disclosed in Analytical Chemistry, Vol. 55, No. 3, pages 477-481.
The conventional APIMS cannot be practically arranged in a plurality of measurement sites in a clean room due to its requirement for differential pumping using a vacuum pump of large displacement. Further, it is difficult to simultaneously monitor the gas purity at various points of the gas supply system.
In the conventional plasma chromatography apparatus, a sample gas is ionized and fed to a drift tube where ions of different species are separated in accordance with the time difference required for the ions to move in the gas in the drift tube under an electric field. In order to analyze a highly-purified gas, the difference between the mobility of main component ions produced by the ionization means and the mobility of impurity ions produced by reaction of the main component ions and the impurity molecules is used to separate the main component ions from the impurity ions in the drift tube. Then, the impurity concentration can be measured from the detected intensity of the impurity ions. This gas analyzer is relatively small in size, and economical, requiring neither a vacuum pump nor a refrigeration system. An example of a plasma chromatography apparatus is disclosed in Analytical Chemistry, Vol. 46, No. 8, pages 710A-720A.
Known plasma chromatography systems have been utilized for analysis of organic components, but not for analysis of water content. More particularly, because the moisture of the carrier gas is subjected to an ionization reaction with organic substances of the impurity, thus acting as a main component ion, the conventional plasma chromatography system does not analyze water content. Moreover, conventional plasma chromatography systems have been incapable of analyzing ultra-low concentrations of water in the highly-purified carrier gas because no consideration has been given to modifying the ion production mechanism in the ion source and in the drift tube, the drift distance, the value of the drift voltage, and the gas purity in the drift tube.